1. Field of Invention
This invention relates to ferromagnetic compositions useful for magnetic recording, and, in particular, to an improved magnetic recording element containing large amounts of nonmagnetic particles to improve physical properties.
2. State of the Art
In general, the magnetic layer of a magnetic recording medium is formed by applying a magnetic coating composition comprising magnetic particles, a binder and an organic solvent onto the surface of a base material such as a polyester film. Such magnetic layer is required to have a high durability since it runs slidingly in close contact with a magnetic head on recording and reproducing of the magnetic recording medium. Further, the magnetic particles in the magnetic layer are required to be uniformly dispersed so as to have a high sensitivity and a good S/N ratio.
The problems associated with friction and wear between magnetic recording elements and recording and playback heads have been frequently dealt with in previous inventions. The problems are particularly severe in computer and television applications where tapes and disks must move at high speeds and the head gaps are shallow. As a result, heads made from the most durable available materials have a short life and a high replacement cost.
One approach to improved head life has been to reduce the abrasiveness of the tape by the incorporation of particulate lubricants such as carbon black and the sulfides of tungsten and molybdenum, as described, for example, in U.S. Pat. Nos. 3,387,993, 3,423,233, 3,437,510, 3,476,596, British Pat. No. 868,346 and German Pat. No. 1,297,671. Another approach to maintaining signal stability is to make head wear more uniform by including in the tape very hard particles that are even more abrasive than the ferromagnetic particles, e.g., flint, garnet, and silicon carbide, as described, for example, in U.S. Pat. No. 3,630,910 and in German Pat. Nos. 1,804,393 and 1,953,459.
Several prior inventions teach the incorporation of various nonmagnetic particulates in magnetic recording elements in various amounts. U.S. Pat. No. 3,929,658 teaches the incorporation of metal oxides with a Moh hardness of 5.5 or less in amounts of 2 to 20% of the weight of the ferromagnetic material. U.S. Pat. No. 3,833,412 discloses the incorporation of abrasive materials with a Moh hardness above 6 and a size below 5 microns. U.S. Pat. No. 4,015,042 is directed to the incorporation of a bimodal distribution of (a) particles with a size of 0.3 to 2 microns in an amount of 0.005 to 0.025, and (b) particles with a size up to 0.1 micron in an amount of 0.02 to 0.06, both amounts being in parts by weight based on the weight of ferromagnetic particles.
In spite of all the prior art teachings, there is still a need for improvement in processing and mechanical properties (durability, adhesion, friction, signal retention, and still framing) of magnetic recording elements in high speed applications.